Airborne particulate matter poses a variety of health and environmental issues. Soot, for instance, caused by combustion (e.g., burning coal, wood, cigarettes, and/or automotive exhaust) may be the primary contaminant in some regions in that it may account for most of the particulate mass present in the air. Despite this, however, individual soot particles can often be relatively small. For instance, soot particles may be smaller than one micron in diameter.
Previous approaches to detecting particulate matter may count particles as they scatter laser light. The scattered light from each particle may be counted by a detector as the particles pass through the laser beam near the detector. However, these approaches may rely on precise airflow control to measure count rate in order to determine the particle number density. Moreover, the 90-degree scattered light detected in previous approaches may only be strong enough such that only particles that are about a micron in diameter or larger are counted. In these approaches, the Mie scattering of smaller particles may be too weak to be detected.
In order to estimate the true particle air mass, these previous approaches may use the count of particles multiplied by a scale (or correction) factor. However, because the actual air mass is so undercounted when counting particles, the scale factors used may need to be large (e.g., on the order of twenty times the counted particle mass). Thus, in order to determine an air mass of micrograms per cubic meter, previous approaches may rely on a scale factor that is undesirably large.